Separate battery banks and inverter

[48Rob]

As seen in the attached drawing, my RV has two separate battery banks.

Battery bank #2 is connected to bus bars that feed the inverter with very hefty 4/0- 18 inch long cables.

The SCC and 120 chargers are also connected to the bus bars. The bus bars also feed a 6 AWG cable that runs to a position one and position 2, switch that feeds the RVs fuse panel.



A second battery bank is located 18 feet from the bus bars and is connected to the selector switch with 6 AWG cable

This setup allows battery bank 2 to act as a backup. Bank 2 is currently a lead acid battery, charged by the vehicle alternator.

I wish to replace it with one or more LFP batteries, and through the use of a selector switch that allows 1, 2, or both, charge them with the existing solar and shore power sources.

I do not have space to put all the batteries in one spot near the inverter.

This setup should charge both battery banks together (albeit more slowly).

The two concerns are that if one bank is run down and the other is not and I switch to both to charge there would be a rush of current from high to low?

Second is “if” I decided to turn on the inverter while the selector switch was turned to “both” (while charging or not) wouldn’t the inverter try to draw from bank 2, through the 6 AWG wire and potentially cause really bad things to happen when the nearest bank to the inverter ran low?

The simple answer it seems is to run much larger cable, but that is impractical both from a cost standpoint, and the fact that the flow from both sides runs through a fuse panel.

A selector switch inline on the charging cable to direct current to bank one, or bank two through a separate cable could work, but requires me to be there to switch over when one reaches full capacity.



Ideas?

 

[rodrick]

Your biggest problem is if 1 battery is real low and you connect to a fully charged battery you will have problems with the bms on the batteries they need to be the same voltage when connected

 

[48Rob]

Thank you.

So keeping them separated at all times is the answer.
Any thoughts on how to charge both banks from one source without combining them?

 

[brainbone]

So keeping them separated at all times is the answer.

IDK, I think it would be easier to just get them both charged up, join them and keep them joined. Not ideal having them separated by so much distance and being different capacities, but it will work. Just make sure the cables between them are sized and fused correctly. (upgrade your 6 AWG from bank 1 to something much larger.)

Any thoughts on how to charge both banks from one source without combining them?

This can get costly, complicated, less efficient, etc., with not much gain.

Edit: I missed this part:

The simple answer it seems is to run much larger cable, but that is impractical both from a cost standpoint, and the fact that the flow from both sides runs through a fuse panel.

The cost of the cable is probably lower than multiple charge controllers, plus you have the added benefit of a larger combined battery bank.

You wouldn't run the larger cable through the fuse panel -- you'd run it directly to the other battery bank.

 

[justinm001]

I've always confused by this but my understanding is power will feed and pull into both banks pretty equal regardless of soc. So 6awg is good for only 55amps meaning anything over 110amps when set to both will cause issues.

Obviously to prevent bad things from happening you just need a 50-60 amp fuse.

If I were you I'd just run a fuse on both sides of the cable and carry a bunch of spare fuses for when you forget to switch from both when running the inverter. You also might want to put the 1/2/both switch on the inverter side so you could run DC off the outside bank then AC off the inside bank. Connect them both when you're certain you'll be under 100a. This way if you forget to switch it you're still charging everything

 

[justinm001]

Your biggest problem is if 1 battery is real low and you connect to a fully charged battery you will have problems with the bms on the batteries they need to be the same voltage when connected

I know this is an issue with leadacid but what specifically happens with LFP? I've done this many times and never any issues. But typically they're charging so it doesn't really matter

 

[justinm001]

Thank you.

So keeping them separated at all times is the answer.
Any thoughts on how to charge both banks from one source without combining them?

Argodiode Battery Isolators - Victron Energy

Make simultaneous charging of two or more batteries from one alternator possible, without connecting the batteries together. With Victrons Argodiode Battery Isolators.

www.victronenergy.com

 

[48Rob]

Thanks for the thoughts, I will try to digest them.

If weight distribution weren't an issue in the travel trailer, I would put all the batteries in one place and be done.
The remote location keeps the balance correct, and safe for towing.
18-20' of heavy cable is hundreds...

The isolator seems like a promising thought. I started to ponder a solenoid with an inverter switch to turn it off/on, removing the inverter from the system until needed/safe but had to get back to work...
The cable from the bus bar to the inverter/inverter draw is the only issue in this whole plan to charge the remote bank.
Remove the inverter and both banks can be tied together (with proper fuses) and life is good. There are no high amp draws in the system except the inverter, which is used infrequently.
Maybe a manual switch in the SCC line to the batteries is a far less expensive option.

 

[HRTKD]

Rob, I have a similar trailer situation. I chose to isolate the battery on the tongue. There is no switch. The tongue battery is used for starting the generator, running the tongue jack and the trailer breakaway system. The tongue battery is charged using a Victron Orion XS 12-12 from the house system. At this time, nothing on the trailer receives a charge from the truck.

My house battery is 48 volt, so I have a Victron Orion Tr 48-12/30 step down converter. The Orion XS hangs off of that.

So far, this is working great. It's clean and simple.

I started off with a lead acid battery on the tongue but recently switched that out to a dinky little NOCO NLP-20. I needed to shed some weight from the tongue. I added warming pads and a thermostat to the NLP-20 for winter.

 

[rodrick]

I know this is an issue with leadacid but what specifically happens with LFP? I've done this many times and never any issues. But typically they're charging so it doesn't really matter

The voltage difference can cause a surge possibly damaging the bms same reason you use a pre charge resistor on a inverter when first powering up

 

[justinm001]

The voltage difference can cause a surge possibly damaging the bms same reason you use a pre charge resistor on a inverter when first powering up

Surge from one battery to the other? Are you saying if I connect a battery at 20% to one at 100% it'll charge? We're talking 12.9v vs 13.4v it's not that big of a difference. I wouldn't think 13.4v charger wouldn't even do anything.

On my DC/DC converters I need to set them to like 14v and even then they only charge to like 70-80%. If I lower the voltage it lowers the percent I charge at. I have them staggered a bit so one stops at like 60% and other closer to 80%.

Inverters have massive capacitors at 0 volts and there's a surge to charge those when powering up. Capacitors are designed to charge and discharge rapidly.

I think when people say this they're referring to the lead acid issues or they're connecting a dead battery to one 100%. I can see if you're down to 10-11v it causing issues

 

[brainbone]

We're talking 12.9v vs 13.4v it's not that big of a difference. I wouldn't think 13.4v charger wouldn't even do anything.

A partially depleted lifepo4 is going to surge a fair bit of current from the full one. Unlike a charger, that current won't really be limited, depending on the BMS, potentially pushing it past its limits. It's best to have the SOC of both much closer when connecting them.

It's a similar issue to why connecting two batteries via 6awg could end badly. Initially, the smaller wire will naturally limit the current pulled from the remote battery as its voltage sags when the inverter pulls higher load, pulling most of that load from the local battery. The difference in SOC between the two banks will diverge, eventually pushing that poor little 6awg past its limits.

 

[Aussieguy]

surge is probably the wrong word ...

if the 2 batteries are at different levels the power transfer from ONE to the OTHER can be very high
this process, can take a fair amount of time..
Putting a lot of strain on BMS components

If it were my system.... and the batteries are same voltage
KEEP them connected so they charge / discharge together ... one bank might charge a bit faster BUT it will even out in the long run

get correct size cables to do the paralleling

I see no reason to keep a bank in reserve .... only adds complexity
if you go through both banks overnight ... you need a bigger bank

 

[brainbone]

surge is probably the wrong word ...

Yeah, not sure why I used that instead of simply "draw." Maybe pre-coffee brain farts.

get correct size cables to do the paralleling

I see no reason to keep a bank in reserve .... only adds complexity
if you go through both banks overnight ... you need a bigger bank

Yep. This is the way. Bite the bullet, run new cable and do it right.

 

[HRTKD]

If weight distribution weren't an issue in the travel trailer, I would put all the batteries in one place and be done.
The remote location keeps the balance correct, and safe for towing.

I understand the need to maintain front/back balance in a travel trailer to ensure there is enough weight on the tongue. That said, if a battery can tip that balance then it sounds like you're on the edge to start with. Switching to LiFePO4 batteries will reduce the weight of battery bank #2 which means slightly more tongue weight, which is better for safe towing but your tow vehicle may not like it.

440 Ah of LiFePO4 is a lot of power in a travel trailer. I had 560 Ah in my prior travel trailer and I don't think I ever went below 70% state of charge in normal use. I don't consider running the A/C after dark as normal use. You didn't say how many Ah of lead acid batteries you have now. If you're swapping 440 Ah of lead for 440 Ah of LiFePO4 then you're doubling the available power because normal practice is to never take lead acid batteries anywhere below 50% state of charge. LiFePO4 can go down to almost 0% state of charge and not complain.

 

[justinm001]

surge is probably the wrong word ...

if the 2 batteries are at different levels the power transfer from ONE to the OTHER can be very high
this process, can take a fair amount of time..
Putting a lot of strain on BMS components

If it were my system.... and the batteries are same voltage
KEEP them connected so they charge / discharge together ... one bank might charge a bit faster BUT it will even out in the long run

get correct size cables to do the paralleling

I see no reason to keep a bank in reserve .... only adds complexity
if you go through both banks overnight ... you need a bigger bank

Sure but how many volts above resting is needed to charge LFP? It's gotta be above .5 volts ..right?

 

[48Rob]

Thanks for the discussion, it is helpful.


I could have given more details in my original post, but didn’t believe them to be relevant. I will correct that now;


The weight balance issue I am working with is not tongue weight but rather too much weight behind the axles.

The travel trailer has a rear, outdoor accessory kitchen which we utilized for the battery storage/DC electrical system. We removed the extra fridge and used that space to our advantage.




As the photo’s show, we installed two hefty 220 Amp hour batteries, and equipment, which is behind the axles. The removal of the fridge saved 30 pounds or so, moving the bumper mounted spare tire to a point under the tongue took away another 40-50 pounds. In the bathroom behind the kitchen wall I removed the glass shower walls/door for another 20 pounds for a total reduction of factory placed weight of around 100 pounds.

This in my mind allowed safe installation of the roughly 160-170 pounds of electrical equipment, wires, etc. We are using Lifepo4 batteries.

The large storage cabinet above the kitchen area and the generous bathroom stuff (that we utilized) added more imbalances which made us tongue light.

I was at 10% tongue weight with this configuration. I added a 35 gallon auxiliary fresh water tank under the front positioned bed, and using weights and scales determined that I could add a couple hundred more pounds of batteries (under the bed) in front to get the tongue weight to 15%-15%. I am towing a 26’ long 6500 pound trailer with an F-350, so overall weight and a little higher tongue weight isn’t an issue.

The original house battery, a lead acid, is still mounted on the tongue and is charged by the truck. It powers the breakaway switch, the power jack, and the slide. No plans to move or change it. We considered switching to an LFP, but it just becomes an easy target for theft. The 118 A/H lead acid also feeds the camper fuse panel via an either/or switch for a little extra reserve (It does not connect to the inverter)

We have 1200 Watts of roof mounted panels; they along with 440 Amp hours of battery are a pretty good combination, and I expect more than adequate in the western states. In the Midwest, shaded campsites are the norm. A couple days of clouds/rain, and little sun getting through the canopy had me trying for 6 days reserve, which we have if we limit inverter items (microwave, coffee pot, toaster, etc.)

We camp a lot in national parks and areas where there are no hookups, and little sun.

We have a regular rooftop A/C, but also installed a through the wall window unit on a slide so that if we are careful, on a hot day the unit can be run for 3-4 hours each day for a couple hot days via the inverter. Having an extra 280 Amp hour battery or two as reserve would be nice. We are drawing around 28-30 Amp hours per hour of A/C use.

Based on fall usage numbers with the furnace being used part time, and NO A/C use we are averaging 72 Amp hours use per day

Ripping it all out and putting everything in front under the bed may be the most practical (right after using electric campsites exclusively) but I’m not ready to jump off that cliff yet…

I have a 120 charger already that I could use to independently charge the new remote bank, that could be powered by the house inverter, shore power, or truck mounted inverter.

My goal was and is to use the excess energy from the solar panels when available for that extra bank.



Rob

 

[brainbone]

My goal was and is to use the excess energy from the solar panels when available for that extra bank.

Maybe I'm not fully grasping what you're after, but the problem is that with simple diodes to isolate charging both banks, you'll charge both at the same time, so if your main bank is the priority, you're kind of wasting the charge to the second remote bank. With a manual switch, you're prone to leaving on the wrong bank when it matters. Also, that remote bank isn't really going to be able to run your A/C directly, and wouldn't want to use it to charge your main bank directly either, needing something to limit the current to what the 6awg and BMSs can handle.

So I'm still having trouble seeing where keeping a remote bank charged as a reserve is all that useful when tied in with only 6awg and maybe some diodes and switches. From my point of view, it's either overly complicated, expensive, and prone to failure, or not all that useful.

Since your usage doesn't sound all that extreme and you have a smallish inverter, you probably could get away with paralleling in the remote bank with something less expensive than a long run of 4/0, but if you're going to go through the effort...

 

[HRTKD]

Only 10% tongue weight is something to be concerned about and I'm glad to hear you're being careful about that.

It wasn't clear if you put lead acid or LiFePO4 batteries in the outside kitchen from the start. You said, "we installed two hefty 220 Amp hour batteries." Does the term 'hefty' refer to weight (lead acid) or Ah (LiFePO4)?

My entire system is installed under the bed.

Post in thread '48 Volt System Toy Hauler Build'

Nov 15, 2024

Today I installed the inverter - for the second time. The first time it wouldn't turn on. The capacitors seemed to get charged, but nothing happened. No lights, no action. That was two days before I left for hunting camp. I ended up rewiring at hunting camp so I could run the generator to get 120v AC power. The problem turned out to be a ribbon cable connector that had come loose. The connector was hidden under another connector, so I didn't see it to try and push it in.

Everything is working, but I'm a bit leery of the inverter. It didn't want to turn on with just the battery...

• HRTKD

• 

 

[48Rob]

Maybe I'm not fully grasping what you're after

I may not have worded it clearly, sorry.

As is, the main bank runs the inverter and 12 volt RV system. The remote bank can also run the rv 12 volt system.
The selector switch allows me to use the remote bank for daily/normal use (lights, tv, water heater, fridge, etc).
When morning coffee or air conditioning is needed, the switch prioritizes the main bank that feeds the inverter.
If I screw up and "forget" to flip the switch, no harm is done. I simply deplete the bank that runs the inverter, and have to forego use of the inverter.
However, either of the two banks can still run everything else.

I investigated different cable sizes, and voltage drop due to distance really rules out all but 4/0.
Maybe I need to shop around more, but the average I found was around $10 per foot, at 40' I'm at $400++

I already had the 6 AWG, which is an upgrade from the factory run 8 AWG that fed the fuse panel.
Correction; I had my numbers mixed up from another project. I have 4 AWG run from the remote bank, not 6 AWG. No matter, it still isn't large enough...

with simple diodes to isolate charging both banks, you'll charge both at the same time, so if your main bank is the priority, you're kind of wasting the charge to the second remote bank

I'm not sure I understand?
Charging dual banks will take longer, but generally "when" there is sun, I have a lot of excess. One bank full, or two banks each half full is still the same, except for having more to feed the inverter.

 

[48Rob]

Does the term 'hefty' refer to weight (lead acid) or Ah (LiFePO4)?

Clear as mud, huh?
They are LiFePO4

 

[HRTKD]

Maybe I need to shop around more, but the average I found was around $10 per foot, at 40' I'm at $400++

This is less than $10/foot.

TEMCo WC0487 Welding Cable - 4/0 AWG 40 ft - Black

temcoindustrial.com

Still, I don't think running cable (even 4/0) from the tongue to the rear of the trailer is practical, even if you ignore the cost.

 

[48Rob]

This is less than $10/foot.

TEMCo WC0487 Welding Cable - 4/0 AWG 40 ft - Black​

temcoindustrial.com

Indeed it is!

Thanks